historical | a/anBecause the stress is on the second syllable (another example is "habitual"), in speech there is a chance that the "h" will not be heard (starting the unstressed syllable). This is the reason why "an" is sometimes used before such words as "historic(al)" and "habitual". Both "a" and "an" are equally correct, but using "an" may seem too formal or even pretentious in normal usage.